Question

I’m getting a very strange behavior in MySQL, which looks like some kind of weird bug. I know it’s common to blame the tried and tested tool for one’s mistakes, but I’ve been going around this for a while.

I have 2 tables, I, with 2797 records, and C, with 1429. C references I.
I want to delete all records in I that are not used by C, so i’m doing:

select * from i where id not in (select id_i from c);

That returns 0 records, which, given the record counts in each table, is physically impossible. I’m also pretty sure that the query is right, since it’s the same type of query i’ve been using for the last 2 hours to clean up other tables with orphaned records.

To make things even weirder…

select * from i where id in (select id_i from c);

DOES work, and brings me the 1297 records that I do NOT want to delete.
So, IN works, but NOT IN doesn’t.

Even worse:

select * from i where id not in (
  select i.id from i inner join c ON i.id = c.id_i
);

That DOES work, although it should be equivalent to the first query (i’m just trying mad stuff at this point).
Alas, I can’t use this query to delete, because I’m using the same table i’m deleting from in the subquery.

I’m assuming something in my database is corrupt at this point.

In case it matters, these are all MyISAM tables without any foreign keys, whatsoever, and I’ve run the same queries in my dev machine and in the production server with the same result, so whatever corruption there might be survived a mysqldump / source cycle, which sounds awfully strange.

Any ideas on what could be going wrong, or, even more importantly, how I can fix/work around this?

Thanks!
Daniel

Answer 1

I would guess that you have a row containing a NULL value. IN and NOT IN behave in a way that you might not expect when there are NULL values. From the manual:

To comply with the SQL standard, IN returns NULL not only if the expression on the left hand side is NULL, but also if no match is found in the list and one of the expressions in the list is NULL.

The value of NOT NULL is still NULL, not true as you hoped.

Here’s a demonstration of the behaviour on a simplified example:

CREATE TABLE c (id_i INT NULL);
INSERT INTO c (id_i) VALUES
(1),
(2),
(NULL);

CREATE TABLE i (id INT NOT NULL);
INSERT INTO i (id) VALUES
(1),
(2),
(3);

SELECT * FROM i WHERE id NOT IN (SELECT id_i FROM c); -- returns nothing
SELECT * FROM i WHERE id IN (SELECT id_i FROM c);     -- returns 1,2
SELECT * FROM i WHERE id NOT IN (                     -- returns 3
  SELECT i.id FROM i INNER JOIN c ON i.id = c.id_i
);